The application relates to a high temperature material particularly a high temperature material for a bi-polar plate of a high temperature fuel cell.
A high temperature fuel cell (Solid Oxide Fuel Cell—SOFC) allows direct conversion of chemical energy into electrical energy. The fuel (H2, CH4, CO, etc.) is separated from an oxidation medium (O2, air) by an oxygen ion conducting solid electrolyte (γ-stabilized ZrO2). At an operating temperature of the fuel cell of about 700° C. to 950° C. oxygen ions are transported from the cathode side through the electrolyte to the anode side where they react with the fuel. Because of a charge compensation, electrons flow through the electrolyte in the same direction.
The electrolyte is coated by porous catalytically active electrode materials. Generally, the anode (fuel side) consists of a Ni/Zro2-cermet and the cathode (oxygen side) consists of a perovskite on the basis of La.
To utilize the SOFC technique for electric power generation, several cells must be connected. Therefore, another component is required that is the so-called bi-polar plate, which is sometimes also called interconnector. The bi-polar plate is utilized as a gas supplying connecting part between the individual cells, but also provides the mechanical support structure of the cell.
The bi-polar plate consists of an alloy that must have certain properties. An essential property is high oxidation resistance in the anode and cathode gases at the high operating temperatures of the fuel cell. Furthermore, because of the required physical compatibility with the ceramic components, it must have a low expansion coefficient (about 10×10−6K−1 to 12×10−6K−1). The optimal expansion coefficient depends on the particular cell concept. With anode substrate supported cells, generally somewhat higher expansion coefficients are required than with cells on the basis of an electrolyte foil concept.
Typical construction materials for the interconnector are ferritic chromium steels. The oxidation resistance of these materials is based on the formation of a protective oxide layer on the basis of Cr2O3, which is formed at high temperatures on the surface of the material. These oxide layers, however, do not behave in an optimum manner at the high operating temperatures. They may spall off from the alloy surface and may thus inhibit the gas flow in the gas channels of the bi-polar plate during long term operations. In addition, the thick Cr2O3 layers formed over extended periods of operation detrimentally affect the electric conductivity of the interconnector. Furthermore, with a high oxygen pressure as it exists generally at the cathode side, volatile chromium oxides and/or hydroxides are formed which poison the cathode/electrode interface and reduce the cell efficiency.
DE 195 47 699 A1 discloses a bi-polar plate of a chromium oxide forming alloy with a mixed oxide layer for increasing the conductivity and reducing the vaporization rate.
DE 44 2 624 A1 discloses a method for protecting chromium-containing bodies wherein the bodies are provided with a protective layer of an oxidic chromate.
A disadvantage of the coating procedure however is that the bipolar plates coated in this way are relatively expensive. In addition, the coating layers cannot be healed during operation of the cell and damages remain.
DE 195 46 614 C2 discloses a chromium oxide forming alloy for high temperature applications which is oxidation resistant for long periods. This alloy includes up to 1% of reactive element additives.
It is the object of the present invention to provide a high temperature material, which heals itself when subjected to mechanical damages at its surface. It is furthermore an object of the invention to provide a bi-polar plate for a high temperature fuel cell, which has the advantages mentioned above.